THE PATH towards finding a second Earth is an important one.
It is important since we seem hell bent on trashing this planet.
Ours is the first generation that - using nuclear, chemical and biological weapons - has been able to kill ourselves as a species.
This power does not seem likely to go away.
When species on earth are confined to a single island or small geographical area, extinction is knocking on their door.
Finding places to live beyond the Earth may be one of the best ways to avoid extinction - learning to live together is another.
Assessing habitability of the newly detected Earth-like planets is astronomy's Holy Grail.
And we need to start thinking about this problem as soon as possible. Thinking is relatively cheap compared to colonising the moon or Mars or mounting an interstellar mission.
There are several kinds of planetary habitability. If terrestrial life could survive on a planet, that is one kind of habitability. If only a particular kind of life that may not resemble terrestrial life can survive, that would be another kind.
Understanding habitability will allow us to identify the closest planet most likely to be habitable for humans. This is the first step towards spreading our wings.
The Earth is our home. The nest where we were born. But you don't stay in the nest forever.
So how likely is it that there is some form of extraterrestrial life on other planets?
If we knew what life was we could answer this question with more confidence.
When we're feeling provocative, some of us would say that we have already found ET.
What we can say with some confidence is that whatever the probability, the chances are going up.
That is to say, that the range of environments in which terrestrial life has been found keeps expanding and the range of potentially habitable environments that we know of, also keeps expanding.
The discovery of planets orbiting other stars is a brand new field of research motivating all kinds of new technology to increase our ability to detect more planets.
Almost every new large astronomical telescope or instrument has ''exoplanet detection'' as one of its most important science goals.
The work at the planetary science institute is multi-disciplinary and spans the range from dating meteorites, understanding the earliest environments on Earth, analysing the latest exoplanet data for patterns that can help answer the questions: Is our solar system typical or unique? What is our place in the universe?
All these exoplanet discoveries bring the question ''Are we alone?'' into the realm of science.
The new science of astrobiology is dedicated to trying to answer this question.
I think most people are fascinated by the question. After a little education about what scientists are doing to answer this question, they begin to believe that indeed, we can make real progress toward answering this question.
The review article that we just published on habitability, ''The Habitability of Our Earth and Other Earths: Astrophysical, Geochemical, Geophysical, and Biological Limits on Planet Habitability'', contributes to our on-going efforts to understand whether nearby Earth-like planets are or could be habitable.
With PhD student Aditya Chopra, we are trying to figure out the elemental composition of life and whether the common elemental composition of life can be used to identify the environment in which terrestrial life originated.
With another PhD student we are analysing multi-planet exoplanetary systems to see if the Titius-Bode Relation can be applied to them and enable us to predict the existence of smaller planets that haven't been detected in these systems. Some of the most exciting things my colleagues and I have discovered are: the age distribution of terrestrial planets in the universe; the high frequency of stars with planets; the large volume of the planet Mars that is compatible with liquid water and life; an entropy of the universe 30 times higher than previously thought; and the first observationally based estimate for the amount of dark energy in the universe.
We're scientists, not mathematicians so we don't deal with proof. We deal with evidence.
The evidence we have is still indirect and it has a lot to do with our slowly increasing ability to understand the origin of life on Earth.
Scientists are all sceptics. That's what we get paid to do.
The evidence speaks for itself.
■ Dr Charley Lineweaver is a senior fellow at the Australian National University's Planetary Science Institute in Canberra.
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